The dominating source of mechanically generated noise inside livestock houses is ventilation fans. Demands on a noise criteria for animal houses were raised during the 1970s. According to the animal welfare directive in the Code of Practice issued by the Swedish Board of Agriculture, the mechanically generated noise in livestock buildings is set to a maximum sound level of 65 dBA in the nearness of an animal. The criteria does not take into account how animals tend to apprehend sound at different frequencies.A distinguish tone can be found among all fans which depends on the speed and the number of wings of the fan. The noise generated at frequencies outside the fan tone is caused by turbulence in the airflow. It is experimentally proved that the sound power generated by a fan is proportionally to the 5th power of the flow velocity.Ventilation systems make out an own category in the acoustics. The sound is led from the source to the recipient by the ventilation ducts. The only relevant changes in sound power come from bends, orifices and silencers.Installation of a silencer is an easy and effective way of reducing the sound power from a ventilation fan in animal houses.